This invention relates to rectifier circuits and particularly to full wave rectifier circuits for providing an output signal linearly related to the absolute value of an AC input signal supplied thereto.
The uses of full wave rectifier circuits are well known. An elementary form of full wave rectifier comprises a bridge connection of four diodes. Such an arrangement is inherently capable of wide bandwidth operation and is unconditionally stable but suffers the disadvantage of being highly non-linear for AC input signal levels near the "knee" of the diode characteristic curve. This limits the usefulness of such rectifier to high level signals of relatively limited dynamic range.
In certain applications the dynamic range of the AC input signal may be quite large, say, 60 dB or more. For example, in certain audio noise reduction systems it is required to rectify an audio signal which may range from a few millivolts to a few volts to generate a DC control voltage for operating an expansion or compression amplifier. Rectifier or absolute value circuits capable of linear operation over this signal range generally employ feedback techniques to reduce the effect of diode nonlinearities. See, generally, "APPLICATIONS OF OPERATIONAL AMPLIFIERS, THIRD GENERATION TECHNIQUES" by J. G. Graeme published in 1973 by the McGraw-Hill Book Company. Chapter 4, section 3, of the Graeme textbook (pp. 119-132) describes numerous absolute value circuits in which diodes are employed in the feedback path of operational amplifiers to provide linear rectification of wide dynamic range AC input signals.
One disadvantage of conventional feedback rectifier circuits is that phase compensation is required for stability and this greatly restricts the circuit bandwidth. Two techniques for reducing frequency response limitations due to phase compensation (and slew rate limitations) are described in section 4.3.3 of the Graeme textbook. One is an arrangement for effectively removing the phase compensation during switching transitions of the rectifier circuit. The other is a biasing circuit that reduces the switching transition voltage range. Such arrangements, however, require additional phase compensation capacitors and relatively complex temperature tracking bias networks.
A need exists for a full wave recitifer circuit having the stability and bandwidth advantages of a feedforward rectifier and the dynamic range and linearity advantages of a feedback rectifier. One prior art approach to meeting this need is described in U.S. Pat. No. 4,336,586 of G. K. Lunn which issued June 22, 1982. The Lunn arrangement employs a pair of non-linear but symmetrical full wave rectifiers having substantially identical transfer characteristics. The AC input signal is rectified by one of the non-linear rectifiers and the resultant signal is fed via a current mirror amplifier (CMA) to the input of an amplifier having a feedback path including the other non-linear rectifier. In operation, the nonlinearities of the first rectifier are effectively cancelled by the identical nonlinearities of the second rectifier. The problem with such an approach is that, for cancellation to occur, the two rectifier circuits must have identical transfer characteristics. This is difficult to achieve in practice, particularly where the dynamic range of the input signal is large and where the circuit may be subject to thermal gradients or other differences in operating conditions.